Fifty Years of TTL

Depending on your source, 2014 marks the 50th anniversary of TTL (transistor-transistor logic). Although some say 1961 is the birth year, TI released the 5400 family in 1964 and the 7400 series in plastic two years later. Since TTL is synonymous with 5400 and 7400, why not offer 50th birthday congratulations?

Alert reader Caron Williams notified me that 2014 marks the 50th anniversary of TTL (transistor-transistor logic). Wikipedia says that although TTL was invented in 1961, TI released the 5400 family of IC’s in 1964, and that the 7400 series in plastic came out two years later. Since TTL is practically synonymous with the 5400 and 7400 series, why not offer 50th birthday congratulations?

Remember that the IC was invented just a few years before TTL came out. Other kinds of logic were commonly used, like DTL (diode-transistor logic) and RTL (resistor-transistor logic). In fact, the Apollo guidance computer was made entirely from RTL components, using several thousand identical ICs, each containing a pair of three input NOR gates.

The Apollo guidance computer IC.

The schematic is clear: RTL logic was barely digital. It wasn’t uncommon for RTL users to bias the transistors nearly linearly and use them as amplifiers. But RTL persisted into the 70s, and in the late 60s we were using gobs and gobs of Fairchild parts for Apollo ground support equipment.

The perfect gate can only assume two states, and RTL’s linearity was exactly the opposite of what the digital world strived for. TTL was designed to approach that ideal. Like an op amp, we also want zero output impedance, and many TTL gates used a “totem pole” configuration of transistors to offer plenty of drive and sink capability.

As mentioned, the anonymous experts at Wikipedia believe the 5400 series predated the much more common 7400 family by a couple of years. The main difference between the two was the temperature range: 5400-series devices generally operated from -55C to 125C, while the 7400 parts, housed in inexpensive plastic packages, were rated from 0C to 70C. Though the 5400 family was often called the “military” version, a lot of commercial applications demanded their extended temperature range.

TTL devices were either SSI (small scale integration), which meant one package contained a couple of flip flops or a few gates, or MSI (medium scale integration), which used more complex parts like multiplexors. The 74181 was an example: It contained an ALU that could add, subtract, and do logic operations on two four-bit inputs. The DIP package had 24 pins, which was an enormous number in those days.

By the late 60s most computers had oceans of TTL components. The following picture is the CPU board from a Nova 1200 16-bit minicomputer. There were no MOS memories at the time. Another board held core memory. All of these DIP packages are 7400-series TTL devices.

A Nova logic board. The dimensions are in inches. These boards were enormous.

Components were so expensive that the earliest Novas used one 74181 four-bit ALU. It pushed a word through one nibble at a time.

As time went on many versions of the 7400/5400 devices appeared. . . . This article continues on EE Times' sister site, Embedded.com.

A lot of engineers still use active-low signals in HDL, which drives me nuts. It just adds confusion, and isn't really helpful unless you accidently time-warp back to 1982. Then when they OR them together they'll DeMorganize which makes everything more confusing:

Ahhh the memories.... That was required reading in one of my EE classes at Cal Poly (mid '80s) I had never dreamed there would be so much intrigue and espionage in the design of a computer. I recall that GI was one of the first to utilize PALs.

I believe the origin of the asymmetry traces back to relay logic. Relays were either on or off (subtly different from high or low) and that conditioned the way of thinking about logic. It suited bipolar transistors very well, especially since they could only make NPN on the chip in the early days (to do both would add process steps). Well, they could have made a chip with all PNP but that performed worse then as they still do today, so TTL was all NPN and engineers could happily continue in the mindset of a "1" being switch on, pull down. There was not much concern about capacitance so pull-up current could be low. External wires themselves became extra logic (a bus line served as a "wired NAND" when it had a pull-up resistor).

CMOS was exotic technology requiring the chip to go through extra process steps. The first forms which came out in the early 70s were aimed at low power and almost seemed like magic (they ran on less than a micro-amp!). But even these, initially, had the same asymmetric ability to sink 20mA (a standard which went back to teletypes and relays) with very little pull-up (which was fortunate since the PMOS transistors, like PNP, were lower performance than NMOS). It was not until the late 80s when CMOS was pervasive and wired-NAND style busses finally died in pursuit of higher frequency / lower power operation that CMOS outputs became routinely symmetric and the idea of pull down faded entirely.

I'm pretty sure that version with zener diodes must have been a very late iteration of TTL. I recall zeners being a novelty in the early 70s as discretes, and it probably added complexity for them to learn to make them on the same chip as the transistors.